An electrical machine such as by way of example a turbo generator is disclosed in EP 1 742 330 A1. An electrical machine of this type comprises a rotor that can be driven and is mounted in such manner that it can rotate and said rotor is also known as an armature, said electrical machine also comprises a stator that is permanently fixed and surrounds the rotor. The rotor comprises a cylindrical rotor shaft that widens in the shaft longitudinal direction centrally to form a rotor core. The rotor core is also described as an armature core. An exciter winding that can be influenced by a current is arranged on the rotor core. The stator comprises a stator winding. The rotor shaft is coupled to a drive, in particular to a drive shaft of a turbine, so as to generate electrical energy. In this manner, it is possible to set the rotor in a rotary motion with respect to the permanently fixed stator. If a current flows through the rotor winding, a magnetic rotating field is thus generated and said magnetic rotating field induces an electrical voltage in the stator winding. Turbo generators can generate an electrical power between 100 MW and 1500 MW.
Furthermore, it is known to provide the cylinder sleeve of the rotor core with grooves that extend in the shaft longitudinal direction and are spaced apart with respect to one another in the circumferential direction and it is also known to arrange the exciter windings in these grooves. A plurality of conductor bars are stacked one above the other in the grooves for this purpose and said conductor bars are insulated with respect to one another and extend in the shaft longitudinal direction. With respect to the cylinder sleeve surface of the rotor core, a groove breech wedge is provided and inserted into a profiling so as to secure the conductor bars against the centrifugal forces that prevail while the rotor is rotating. In the case of a turbo generator, the aim is to achieve a high as possible electrical power output. The electrical power output of a turbo generator that can be achieved depends on inter alia the magnetic field strength of the magnetic rotating field that is generated by the exciter winding of the rotor. The higher the magnetic field strength, the higher the corresponding electrical power output of the turbo generator.
Since the conductor bars are intensely heated during operation as a result of the electrical currents, it is necessary to cool said conductor bars. For this purpose, it is known to cool the conductor bars using air, gas or water. The conductor bars that are embodied from a conductive material, in particular copper, are provided by means of bore holes, wherein a cooling medium flows through these bore holes. Radially aligned and also axially aligned cooling paths in the conductor bars are known. By way of example, cooling paths are embodied in an axial or radial direction in such a manner that a slit is milled into the conductor bars that are arranged one on top of the other and a cooling medium can flow through said slits. In order to ensure that the maximum possible amount of heat is transferred, these slits are embodied in a stepped manner so that the cooling medium is subjected to a swirling action at the step and the heat transfer coefficient is consequently increased which results in improved cooling efficiency.
In the case of generator rotors that are cooled radially by means of air or hydrogen, the power losses in the rotor conductors in the active part of the rotor are dissipated to the cooling gas by way of radial cooling slits in the conductors. The dissipation of heat is limited as a result of the cooling surface and the cooling gas flow. The conducting temperatures increase in the active part in the groove from below upwards. The hottest conductor lies in the upper region of the groove and limits the permitted rotor current.
It is known to increase the swirling effect of the cooling gases by means of constructing steps in the conductors so as to improve the dissipation of heat in the radial cooling ducts of a radially cooled generator rotor.